Strength training effects in prepubescent boys

Motor unit firing rates increase in prepubescent youth following linear periodization resistance exercise training

PURPOSE

The purpose was to examine the effects of 8-weeks (3 days/week) of linear periodization resistance exercise training (RET) on neuromuscular function in prepubescent youth.

METHODS

Twenty-five healthy prepubescent youth (11 males, 14 females, age = 9.1 ± 0.8 years) completed the RET (n = 17) or served as controls (CON, n = 8). Isometric maximal voluntary contractions (MVCs) and trapezoidal submaximal contractions at 35 and 60% MVC of the right leg extensors were performed with surface electromyography (EMG) recorded from the leg extensors [vastus lateralis (VL), rectus femoris, and vastus medialis] and flexors (biceps femoris and semitendinosus). EMG amplitude of the leg extensors and flexors were calculated during the MVCs. Motor unit (MU) action potential trains were decomposed from the surface EMG of the VL for the 35 and 60% MVCs. MU firing rates and action potential amplitudes were regressed against recruitment threshold with the y-intercepts and slopes calculated for each contraction. Total leg extensor muscle cross-sectional area (CSA) was collected using ultrasound images. ANOVA models were used to examine potential differences.

RESULTS

Isometric strength increased post-RET (P = 0.006) with no changes in leg extensor and flexor EMG amplitude. Furthermore, there were no changes in total CSA or the MU action potential amplitude vs. recruitment threshold relationships. However, there were increases in the firing rates of the higher-threshold MUs post-RET as indicated with greater y-intercepts (P = 0.003) from the 60% MVC and less negative slope (P = 0.004) of the firing rates vs. recruitment threshold relationships at 35% MVC.

CONCLUSIONS

MU adaptations contribute to strength increases following RET in prepubescent youth.

Resistance training effects on pubertal children with a risk of developing pediatric dynapenia

PURPOSE

Many modern-day children are at risk of pediatric dynapenia (muscle weakness). We examined the effects of a 12-week resistance training (RT) program on neuromuscular function and body composition parameters in pubertal children with a risk of dynapenia.

METHODS

Twelve children (13.4 ± 0.9 y) with dynapenia performed a progressive RT program consisting of knee extension and flexion, bench press, abdominal crunch, back extension, lateral pull-down, elbow flexion, and upright row (1-2 sets of 10-15 repetitions/exercise) twice/week for 12 weeks. Outcome measures included one-repetition maximum (1-RM) strength, maximal voluntary isometric contraction (MVIC) torque, rate of torque development (RTD), electromyographic (EMG) activity, muscle thickness (MT), muscle quality (MQ) assessed by echo intensity (MQEI) of the knee extensors and specific tension of MVIC torque to thigh fat-free mass (MQST), and total and regional body and bone composition assessed by dual-energy X-ray absorptiometry. Changes in the measures before and after the 12-week RT and associations among the measures were analyzed by linear mixed models.

RESULTS

Significant (p < 0.05) increases in 1-RM (63.9 ± 4.5%), MVIC torque (16.3 ± 17.8%), MT (18.8 ± 5.5%) and MQ (MQEI: -25.9 ± 15.2%; MQST: 15.1 ± 18.8%;) were evident from pre- to post-training. Total fat-free mass (FFM) increased by 2.3 ± 3.2% from baseline (p = 0.01), but no changes (p > 0.05) in the other measures were observed. Significant (p < 0.05) associations between the changes in 1-RM and/or MVIC torque and the changes in quadriceps MT, MQEI, MQST and total body FFM were evident.

CONCLUSIONS

The 12-week RT was effective for improving neuromuscular and body composition parameters, and thereby reversed the risk of pediatric dynapenia.

Combined Physical Training Strategies Improve Physical Fitness, Behavior, and Social Skills of Autistic Children

The present study aimed to investigate the effects of combined physical training (CPT) on social skills and physical fitness (PF) in children with ASD. Sixteen children with autism (age 6-10 years) were randomly assigned into two groups: CPT and control. The CPT group participated in a program involving ball game, rhythmic movements, and resistance training for eight weeks (three sessions per week). PF and behavior profile were assessed before and after training. CPT program had a significant effect on indicators of social skills such as stereotypic behavior and communication, as well as PF such as handgrip strength, upper and lower body power, flexibility, balance, and agility (P < 0.05). CPT in autistic children can improve indicators of social skills and PF.

Sport-Related Effect on Knee Strength Profile during Puberty: Basketball vs. Soccer

The present study examined and compared the isokinetic peak torque and the reciprocal ratios of the knee joint between young basketball and soccer players. An amount of 100 soccer and 100 basketball players took part in this study and were separated into five equal groups (n = 20), according to their chronological age (12, 13, 14, 15, and 16 years old). The absolute concentric (CON) and eccentric (ECC) peak torque of the knee flexor and extensor muscles (at 60°/s, 180°/s) were assessed using a Cybex Norm dynamometer, and the relative peak torque (per unit of body mass), as well as the conventional (CON/CON; ECC/ECC) and functional (CON/ECC; ECC/CON) ratios, were calculated. Data analysis indicated that the basketball players had higher absolute peak torque values than the soccer players throughout their developmental ages (p < 0.05). When the isokinetic peak torque values were normalized relative to body mass, no differences were observed between basketball and soccer players in any age group (p > 0.05). Additionally, no differences were observed in conventional and functional ratios between soccer and basketball players (p > 0.05). In conclusion, it appears that, during developmental ages (12-16 years old), the isokinetic strength profile (independent of body mass affecting absolute values) of knee extensor and flexor muscles develops similarly in basketball and soccer players.

Meta-Analysis of the Effects of Plyometric Training on Lower Limb Explosive Strength in Adolescent Athletes

BACKGROUND

Plyometric training is an effective training method to improve explosive strength. However, the ability to perform plyometric training in the adolescent population is still controversial, with insufficient meta-analyses about plyometric training on lower limb explosive strength in adolescent athletes.

OBJECTIVE

To investigate the influence of plyometric training on the explosive strength of lower limbs in adolescent athletes.

METHODS

We performed a search of six databases (Web of Science, PubMed, Scopus, ProQuest databases, China National Knowledge Infrastructure (CNKI), and Wan-fang database) from the starting year of inclusion in each database to April 4, 2022. The quality of the included literature was assessed using the Cochrane risk assessment tool, and data were analyzed using the Review Manager 5.4 software.

RESULT

Plyometric training had significant effects on the performance of adolescent athletes in countermovement jump (MD = 2.74, 95% CI: 1.62, 3.85, p < 0.01), squat jump (MD = 4.37, 95% CI: 2.85, 5.90, p < 0.01), standing long jump (MD = 6.50, 95% CI: 4.62, 8.38, p < 0.01), 10-m sprint (MD = -0.04, 95% CI: -0.08, -0.00, p = 0.03), and 20-m sprint (MD = -0.12, 95% CI: -0.20, -0.04, p = 0.03); all had positive and statistically significant effects (p < 0.05).

CONCLUSION

Plyometric training can significantly enhance the explosive strength of lower limbs in adolescent athletes.

Cardiorespiratory fitness, muscle fitness, and physical activity in children with long QT syndrome: A prospective controlled study

Background

In children with congenital long QT syndrome (LQTS), the risk of arrhythmic events during exercise commonly makes it difficult to balance exercise restrictions versus promotion of physical activity. Nevertheless, in children with LQTS, cardiorespiratory fitness, muscle fitness, and physical activity, have been scarcely explored.

Materials and methods

In this prospective, controlled, cross-sectional study, 20 children with LQTS (12.7 ± 3.7 years old) and 20 healthy controls (11.9 ± 2.4 years old) were enrolled. All participants underwent a cardiopulmonary exercise test, a muscular architecture ultrasound assessment, (cross-sectional area on right rectus femoris and pennation angle), a handgrip muscular strength evaluation, and a standing long broad jump test. The level of physical activity was determined using with a waist-worn tri-axial accelerometer (Actigraph GT3X).

Results

Peak oxygen uptake (VO_2peak) and ventilatory anaerobic threshold (VAT) were lower in children with LQTS than in healthy controls (33.9 ± 6.2 mL/Kg/min vs. 40.1 ± 6.6 mL/Kg/min, P = 0.010; 23.8 ± 5.1 mL/Kg/min vs. 28.8 ± 5.5 mL/Kg/min, P = 0.007, respectively). Children with LQTS had lower standing long broad jump distance (119.5 ± 33.2 cm vs. 147.3 ± 36.1 cm, P = 0.02) and pennation angle (12.2 ± 2.4° vs. 14.3 ± 2.8°, P = 0.02). No differences in terms of moderate-to-vigorous physical activity were observed (36.9 ± 12.9 min/day vs. 41.5 ± 18.7 min/day, P = 0.66), but nearly all children were below the WHO guidelines.

Conclusion

Despite similar physical activity level, cardiorespiratory fitness and muscle fitness in children with LQTS were lower than in healthy controls. The origin of this limitation seemed to be multifactorial, involving beta-blocker induced chronotropic limitation, physical and muscle deconditioning. Cardiovascular rehabilitation could be of interest in children with LQTS with significant physical limitation.

Regulatory Mechanisms of Muscle Mass: The Critical Role of Resistance Training in Children and Adolescent

Muscle mass and strength are subjected to several regulations. We found endocrine signals such as growth hormone, insulin-like growth factor 1, testosterone, thyroid hormones, and glucocorticoids among them. Neural inputs also influence muscle development, modulating mass and strength. Among the external stimuli that modulate these muscular features is physical training such as resistance and endurance training. Specifically, resistance training can mediate an increase in muscle mass by hypertrophy in adults, but the effects in children and adolescents are full of myths for most of the population. However, the evidence shows that the impact of resistance training on children and adolescents is clear and provides a wide range of benefits. However, qualified professionals must be available since exercise prescription and subsequent supervision must follow this population's abilities, needs, and interests.

Asimetría entre los miembros superiores en jóvenes padelistas de competición

Existe escasa literatura sobre las asimetrías en el pádel, un deporte de naturaleza asimétrica. Para estudiar el desarrollo de asimetrías en este deporte se evaluó, con bioimpedancia, la masa magra de ambos miembros superiores de 96 jugadores de pádel jóvenes y de 76 esquiadores (grupo control) y se calculó el índice de simetría. Los jugadores de pádel tuvieron una asimetría entre ambos miembros superiores mayor que los esquiadores, cuando se consideró toda la muestra (7.2 ± 5 % vs. 1.4 ± 3.2 %; p < 0.001), en jugadores con un estado de madurez negativo (5,7 ± 3,2 % vs. 1,5 ± 3,8 %; p < 0,001) y en aquellos con un estado de madurez positivo (8,3 ± 5,8 % vs. 1,3 ± 2,4 %; p < 0,001). Este estudio revela que el pádel genera asimetrías de masa magra en los miembros superiores, incluso antes de la edad de crecimiento pico.

A juvenile climbing exercise establishes a muscle memory boosting the effects of exercise in adult rats

AIM

Investigate whether juvenile exercise could induce a long-term muscle memory, boosting the effects of exercise in adults.

METHODS

We devised a 5-week climbing exercise scheme with food reward administered to male juvenile rats (post-natal week 4-9). Subsequently, the animals were subjected to 10 weeks of detraining (week 9-19) without climbing and finally retraining during week 19-21.

RESULTS

The juvenile exercise increased fiber cross-sectional area (fCSA) by 21% (p = 0.0035), boosted nuclear accretion by 13% (p = 0.057), and reduced intraperitoneal fat content by 28% (p = 0.007) and body weight by 9% (p = 0.001). During detraining, the fCSA became similar in the animals that had been climbing compared to naive controls, but the elevated number of myonuclei induced by the climbing were maintained (15%, p = 0.033). When the naive rats were subjected to 2 weeks of adult exercise there was little effect on fCSA, while the previously trained rats displayed an increase of 19% (p = 0.0007). Similarly, when the rats were subjected to unilateral surgical overload in lieu of the adult climbing exercise, the increase in fCSA was 20% (p = 0.0039) in the climbing group, while there was no significant increase in naive rats when comparing to the contralateral leg.

CONCLUSION

This demonstrates that juvenile exercise can establish a muscle memory boosting the effects of adult exercise. The juvenile climbing exercise with food reward also led to leaner animals with lower body weight. These differences were to some extent maintained throughout the adult detraining period in spite of all animals being fed ad libitum, indicating a form of body weight memory.

Progressive resistance training for children with cerebral palsy: A randomized controlled trial evaluating the effects on muscle strength and morphology

Children with spastic cerebral palsy often present with muscle weakness, resulting from neural impairments and muscular alterations. While progressive resistance training (PRT) improves muscle weakness, the effects on muscle morphology remain inconclusive. This investigation evaluated the effects of a PRT program on lower limb muscle strength, morphology and gross motor function. Forty-nine children with spastic cerebral palsy were randomized by minimization. The intervention group (nparticipants = 26, age: 8.3 ± 2.0 years, Gross Motor Function Classification System [GMFCS] level I/II/III: 17/5/4, nlegs = 41) received a 12-week PRT program, consisting of 3-4 sessions per week, with exercises performed in 3 sets of 10 repetitions, aiming at 60%–80% of the 1-repetition maximum. Training sessions were performed under supervision with the physiotherapist and at home. The control group (nparticipants = 22, age: 8.5 ± 2.1 year, GMFCS level I/II/III: 14/5/3, nlegs = 36) continued usual care including regular physiotherapy and use of orthotics. We assessed pre- and post-training knee extension, knee flexion and plantar flexion isometric strength, rectus femoris, semitendinosus and medial gastrocnemius muscle morphology, as well as functional strength, gross motor function and walking capacity. Data processing was performed blinded. Linear mixed models were applied to evaluate the difference in evolution over time between the control and intervention group (interaction-effect) and within each group (time-effect). The α-level was set at p = 0.01. Knee flexion strength and unilateral heel raises showed a significant interaction-effect (p ≤ 0.008), with improvements in the intervention group (p ≤ 0.001). Moreover, significant time-effects were seen for knee extension and plantar flexion isometric strength, rectus femoris and medial gastrocnemius MV, sit-to-stand and lateral step-up in the intervention group (p ≤ 0.004). Echo-intensity, muscle lengths and gross motor function showed limited to no changes. PRT improved strength and MV in the intervention group, whereby strength parameters significantly or close to significantly differed from the control group. Although, relative improvements in strength were larger than improvements in MV, important effects were seen on the maintenance of muscle size relative to skeletal growth. In conclusion, this study proved the effectiveness of a home-based, physiotherapy supervised, PRT program to improve isometric and functional muscle strength in children with SCP without negative effects on muscle properties or any serious adverse events.

Clinical Trial Registration: ClinicalTrials.gov, identifier NCT03863197.

A randomised controlled trial of 1- versus 2-day per week formats of Nordic hamstring training on explosive athletic tasks in prepubertal soccer players

This randomised controlled trial examined the effect of volume-equated programmes of Nordic hamstring exercise (NHE) training, executed at frequencies of 1- or 2-days per week, on explosive athletic tasks (30 m sprint, 15 m manoeuvrability and standing long jump [SLJ]) in male youth soccer players (mean age: 10.3 ± 0.5 years). Players were divided into an experimental group (n = 31) which was further subdivided into 1-day (n = 16) and 2-days (n = 15) per week training conditions, and a control group (n = 14). There were significant group-by-time interactions for 30-m sprint (p < 0.001, d = 0.6), SLJ (p = 0.001, d = 1.27) and 15 m manoeuvrability (p < 0.001, d = 0.61). The experimental group demonstrated small to moderate effect sizes in 30-m sprint (d = 0.42, p = 0.077), SLJ (d = 0.97, p < 0.001) and 15 m manoeuvrability (d = 0.61, p < 0.001). The control group showed small significant performance decrements or no change in these variables. There were no significant differences between the 1-day and 2-day training groups. In two of the three tests (30 m sprint, SLJ) the 2-day group demonstrated larger effect sizes. The NHE enhances explosive athletic task performance in prepubertal youth soccer players and there may be only small advantages to spreading training over two days instead of one.

Effects of school-based neuromuscular training on fundamental movement skills and physical fitness in children: a systematic review

Objectives

The primary purpose of this review was to clarify the effects of school-based integrated neuromuscular training (INT) on fundamental movement skills and physical fitness in children. The secondary purpose was to examine whether school-based INT intervention is superior to physical education (PE) intervention in enhancing motor skills and fitness.

Methods

A systematic literature search was performed in four electronic databases: PubMed, Web of Science, MEDLINE (EBSCOhost), and Cochrane Central Register of Controlled Trials. The last search was performed on December 21, 2021, and was limited to the English language, human species, and peer reviewed journals. Randomized controlled trials and cluster randomized controlled trials that examine the effects of school-based INT on motor skills and/or fitness in healthy children who were aged up to 14 years old were included. Moreover, studies included in this study should compare school-based INT-induced adaptions with those generated by PE interventions. Studies that involve athletic children and additional exercise training were excluded. The Physiotherapy Evidence Database (PEDro) scale was used to assess the quality of the study.

Results

Of 1,026 studies identified, seven original trials that meet the inclusion criteria were included in this review. Based on the PEDro scale, the PEDro score of seven studies was between six and eight points with a mean score of 5.29. Among the seven studies included in this study, four studies assessed physical fitness including muscular fitness (n = 4), speed (n = 3), endurance (n = 2), and flexibility (n = 2). Three studies examined the effects of INT on postural control and three studies explored its effects on motor skills. Concerning movement competence, significant and greater improvements in postural control and fundamental motor skills were observed following school-based INT interventions compared to PE intervention in two and three studies, respectively. Regarding physical fitness, neuromuscular training significantly increased muscular fitness, speed, endurance, flexibility in three, two, one, and one studies, respectively. However, only greater improvements in muscle fitness were observed in school-based INT group compared to PE group. The main limitations of this review were the lack of descriptions of training intensity and volume and the low methodological quality of the included studies.

Conclusion

This review provides evidence that school-based neuromuscular training programs are superior to PE lessons in improving postural control, fundamental motor skills and muscular strength. Therefore, INT could be incorporated into traditional physical education classes in school. Trial registration number: CRD42022297349.

Effects of Strength Training on Body Fat in Children and Adolescents with Overweight and Obesity: A Systematic Review with Meta-Analysis

Childhood overweight and obesity represent a growing public health problem worldwide. Since the 1980s, the global prevalence of overweight and obesity in childhood and adolescence has increased by 47%. The promotion of exercise is an important intervention to reduce the physical damage of obesity. The meta-analysis was conducted in accordance with the general guidelines for the reporting of systematic reviews and meta-analyses (PRISMA). The PubMed, SciELO, ScienceDirect and Google Scholar databases were searched from August to December 2021. The search yielded 722 titles published between 2000 and 2021. After screening the titles and abstracts, 64 duplicate articles were detected, and 27 articles were ultimately included in the systematic review, including 26 articles published in English and one published in Spanish. There was a statistically significant effect of the strength training interventions on the percentage of body fat, Test of 0 i = (p = 0.00, z = 6.92), Test of 0 = (p = 0.00, Q (9) = 42.63). The findings reveal that strength training has a positive impact on the treatment of body fat in children and adolescents with overweight and obesity.

Neuromuscular characteristics of front and back legs in junior fencers

In elite fencers, muscle strength and muscle mass of the front leg (FL) are greater than those of the back leg (BL) due to characteristic physiological and biomechanical demands placed on each leg during fencing. However, the development of laterality in their neural and muscular components is not well-understood. The present study investigated neuromuscular characteristics of FL and BL in junior fencers. Nineteen junior fencers performed neuromuscular performance tests for FL and BL, separately. There were no significant differences in the isometric knee extension strength (MVC), unilateral vertical jump (UVJ), vastus lateralis muscle thickness (MT), or motor unit firing rate of the vastus lateralis muscle (MUFR) between FL and BL (p > 0.05). In subgroup analyses, a significantly greater MUFR in FL than BL was noted only in fencers with > 3 years of fencing experience, and significantly greater UVJ in FL than BL was observed solely in fencers with < 3 years of fencing experience (p < 0.05). Strong positive correlations between FL and BL were identified in MVC, MT, and MUFR in fencers with > 3 years of fencing experience, but not in those with < 3 years of experience. These findings suggest that in junior fencers, laterality in neuromuscular performance has not manifested, whereas longer fencing experience induces fencing-dependent laterality in neural components, and laterality in dynamic muscle strength is decreased with fencing experience.

Effects of Strength and Conditioning on Maximal Isometric Strength, Motor Unit Behavior, and Concentric Isokinetic Peak Torque in Middle-School Boys'

ABSTRACT

MacLennan, RJ, Mota, JA, Thompson, BJ, and Stock, MS. Effects of strength and conditioning on maximal isometric strength, motor unit behavior, and concentric isokinetic peak torque in middle-school boys. J Strength Cond Res 36(5): 1318-1326, 2022-It has long been theorized that improvements in muscle strength in young athletes are mediated by motor unit adaptations. The ability to decompose surface electromyographic signals obtained during isometric contractions now allow for such research questions to be answered. We examined changes in isometric and concentric isokinetic strength, as well as vastus lateralis motor unit behavior, after 16 weeks of strength training and conditioning in middle-school aged boys. Nine boys (mean ± SD age = 12 ± 1 years) participated in training. Five boys (age = 13 ± 1 years) served as control subjects. The training subjects performed 90 minutes of high-intensity, multi-joint exercise twice per week. Assessments of unilateral maximal voluntary isometric contraction (MVIC) force of the knee extensors, concentric peak torque at velocities of 60, 180, and 300°·s-1, and vastus lateralis motor unit data during 50 and 80% MVIC tests were performed. Strength training and conditioning did not improve MVIC force. Greater training-induced strength increases were observed at faster isokinetic velocities, with a large effect size at 300°·s-1 (d = 0.813). The slopes and y-intercepts of the mean firing rate vs. recruitment threshold relationship and the action potential amplitude vs. recruitment threshold relationship were unaffected by training. Sixteen weeks of middle-school strength and conditioning did not enhance maximal isometric strength or vastus lateralis motor unit control, but improvements were observed during rapid isokinetic muscle actions. Given the lack of training (multi-joint) vs. testing (single-joint) specificity, we propose that motor unit adaptations in youth are task specific.

Increase in Volitional Muscle Activation from Childhood to Adulthood: A Systematic Review and Meta-analysis

INTRODUCTION

Children's maximal muscle strength is consistently lower than adults', even when normalized to body size. Lower volitional muscle activation (VA) in children is often considered one of the main reasons for age-related differences in muscular performance. However, some recent studies have reported similar VA in children and adults, bringing into question whether there is indeed an age-related increase in VA. The purpose of this review was to determine the effect of age on VA during maximal isometric contractions.

METHODS

Literature examining VA differences, using twitch interpolation in children (7-14 yr) and adults (16-28 yr), was systematically reviewed. Of the 1915 studies initially identified, 19 data sets were eligible for inclusion in the qualitative analysis and 14 in the quantitative meta-analysis (comprising 207 children and 193 adults).

RESULTS

Significantly lower VA in children was reported in 9/19 (47%) studies. A random-effects meta-analysis found a strong effect of age on VA, supporting lower VA in children compared with adults (Hedges' g = 1.55; confidence interval: 0.9-2.13). Moderator analysis included muscle group, sex, children's age, stimulation number (singlet, multiple), type (electric, magnetic), and location (muscle, nerve), of which only muscle group was significant (P < 0.001). A significant Egger's regression test and asymmetrical funnel plot suggest that publication bias may be present.

CONCLUSIONS

Overall, these findings suggest that compared with adults, children activate their motor-unit pool less compared with adults. Moreover, that the degree of VA increase with age may be influenced by the muscle examined (upper vs lower extremity). However, more research is needed to elucidate the influence of this possible factor, as the current review contains limited data from upper body muscles. The developmental mechanism responsible for children's lower VA requires further research.

Unique Considerations for the Pediatric Athlete During Rehabilitation and Return to Sport After Anterior Cruciate Ligament Reconstruction

Level of Evidence

Vertical Drop Jump Biomechanics of Patients With a 3- to 10-Year History of Youth Sport-Related Anterior Cruciate Ligament Reconstruction

Background

A better understanding of movement biomechanics after anterior cruciate ligament reconstruction (ACLR) could inform injury prevention, knee injury rehabilitation, and osteoarthritis prevention strategies.

Purpose

To investigate differences in vertical drop jump (VDJ) biomechanics between patients with a 3- to 10-year history of youth sport-related ACLR and uninjured peers of a similar age, sex, and sport.

Study Design

Cross-sectional study. Level of evidence III.

Methods

Lower limb kinematics and bilateral ground-reaction forces (GRFs) were recorded for participants performing 10 VDJs. Joint angles and GRF data were analyzed, and statistical analysis was performed using 2 multivariate models. Dependent variables included sagittal (ankle, knee, and hip) and coronal (knee and hip) angles at initial contact and maximum knee flexion, the rate of change of coronal knee angles (35%-90% of the support phase; ie, slopes of linear regression lines), and vertical and mediolateral GRFs (normalized to body weight [BW]). Fixed effects included group, sex, and time since injury. Participant clusters, defined by sex and sport, were considered as random effects.

Results

Participants included 48 patients with a history of ACLR and 48 uninjured age-, sex-, and sport-matched controls (median age, 22 years [range, 18-26 years]; 67% female). Patients with ACLR demonstrated steeper negative coronal knee angle slopes (β = -0.04 deg/% [95% CI, -0.07 to -0.00 deg/%]; P = .025). A longer time since injury was associated with reduced knee flexion (β = -0.2° [95% CI, -0.3° to -0.0°]; P = .014) and hip flexion (β = -0.1° [95% CI, -0.2° to -0.0°]; P = .018). Regardless of ACLR history, women displayed greater knee valgus at initial contact (β = 2.1° [95% CI, 0.4° to 3.8°]; P = .017), greater coronal knee angle slopes (β = 0.05 deg/% [95% CI, 0.02 to 0.09 deg/%]; P = .004), and larger vertical GRFs (landing: β = -0.34 BW [95% CI, -0.61 to -0.07 BW]; P = .014) (pushoff: β = -0.20 BW [95% CI, -0.32 to -0.08 BW]; P = .001).

Conclusion

Women and patients with a 3- to 10-year history of ACLR demonstrated VDJ biomechanics that may be associated with knee motion control challenges.

Clinical Relevance

It is important to consider knee motion control during activities such as VDJs when developing injury prevention and rehabilitation interventions aimed at improving joint health after youth sport-related ACLR.

A narrative review of skeletal muscle atrophy in critically ill children: pathogenesis and chronic sequelae

Muscle wasting is now recognized as a growing, debilitating problem in critically ill adults, resulting in long-term deficits in function and an impaired quality of life. Ultrasonography has demonstrated decreases in skeletal muscle size during pediatric critical illness, although variations exist. However, muscle protein turnover patterns during pediatric critical illness are unclear. Understanding muscle protein turnover during critical illness is important in guiding interventions to reduce muscle wasting. The aim of this review was to explore the possible protein synthesis and breakdown patterns in pediatric critical illness. Muscle protein turnover studies in critically ill children are lacking, with the exception of those with burn injuries. Children with burn injuries demonstrate an elevation in both muscle protein breakdown (MPB) and synthesis during critical illness. Extrapolations from animal models and whole-body protein turnover studies in children suggest that children may be more dependent on anabolic factors (e.g., nutrition and growth factors), and may experience greater muscle degradation in response to insults than adults. Yet, children, particularly the younger ones, are more responsive to anabolic agents, suggesting modifiable muscle wasting during critical illness. There is a lack of evidence for muscle wasting in critically ill children and its correlation with outcomes, possibly due to current available methods to study muscle protein turnover in children-most of which are invasive or tedious. In summary, children may experience muscle wasting during critical illness, which may be more reversible by the appropriate anabolic agents than adults. Age appears an important determinant of skeletal muscle turnover. Less invasive methods to study muscle protein turnover and associations with long-term outcome would strengthen the evidence for muscle wasting in critically ill children.

The effects of strength vs. plyometric training on sprinting kinetics in post peak height velocity (PHV) female athletes

Speed is a crucial factor for overall athletic development. While researchers have shown strength and plyometric training to improve sprinting speed in some adult and youth populations, no studies have compared the effects of strength and plyometric training on sprinting speed in young females. Fifty-two young females were divided into three groups and trained for 7 weeks, twice a week; strength training (n = 16, age 13.36 ± 0.84), plyometric training (n = 21, age 13.38 ± 0.75) and a physical education class as a control group (n = 15, age 13.95 ± 0.54). Participants were tested for sprinting performance and horizontal force (Fo), maximum velocity (Vmax) and maximum horizontal power (Pmax) metrics over 30 m distance, isometric strength and unilateral horizontal jump distance before and after the intervention. Both the strength and plyometric groups significantly improved all performance variables (p < 0.05). The strength group significantly improved 10 m split time (6.76%; Hedge's g = 0.65) and Fo (18.98%; g = 0.67), whereas the plyometric group significantly improved Vmax (4.91%; g = 0.50) and Pmax (7.91%; g = 0.31). The findings of this study suggest that both strength and plyometric training can improve sprinting kinetics, jumping performance and overall strength in young females.

Effects of Dry-Land Training Programs on Swimming Turn Performance: A Systematic Review

Swimming coaches have prescribed dry-land training programs over the years to improve the overall swimming performance (starts, clean swimming, turns and finish). The main aim of the present systematic review was to examine the effects of dry-land strength and conditioning programs on swimming turns. Four online databases were scrutinised, data were extracted using the Preferred PRISMA guidelines and the PEDro scale was applied. A total of 1259 articles were retrieved from database searches. From the 19 studies which were full-text evaluated, six studies were included in the review process. The review indicated that plyometric, strength, ballistic and core training programs were implemented for improving swimming turn performance. Strength, ballistic and plyometric training focusing on neural enhancement seem to be effective for improving swimming turn performance. The data related to training of the core were not conclusive. Coaches should consider incorporating exercises focusing on improving the neuromuscular factor of the leg-extensor muscles into their daily dry-land training programs. More researches are needed to provide a better understanding of the training methods effects and training organisations for improving swimming turn performance.

Effect of the Growth Spurt on Training of Strength and Power During Mid-Adolescence in Boys

ABSTRACT

Badenhorst, J, Pienaar, AE, and Gerber, BP. The effect of the growth spurt on training of strength and power during mid-adolescence in boys. J Strength Cond Res 35(8): 2193-2204, 2021-This study determined if the growth spurt will influence the training of strength and power in boys (N = 86) of differing maturity levels. Based on a 2-group, pretest posttest design within a 2 school-year follow-up period, including 4 time point measurements, an experimental group (EG), who participated in different school sports were subjected to an additional 6 months, strength and power training program, twice a week for 45 minutes over 2 consecutive years (n = 47, 13.67 ± 0.26 years). They were compared with boys (n = 39, 13.68 ± 0.16 years) serving as a control group (CG) for normal growth who did not compete in any sports or training programs. Both EG and CG were subdivided into 3 maturity groups, late developers (LD), early developers, and typical developers (TD) based on a linear growth rate. Assessments were performed every 6 months for changes in height, body mass, strength, and power. Differences within and between the EG, CG, and maturity groups were analyzed using repeated measures analysis of variance and hierarchical linear modeling, whereas significance of practical differences were determined by effect sizes (d). Early developers presented with training effects, whereas TD and LD showed conditioning and adaptation responses to training. Customized training to leverage the different maturation levels in a way to optimize strength and power advancement is important for the preparation of boys for sports participation during mid-adolescence because it can positively affect adaptation and responsiveness to training in boys of different maturity levels.

The Influence of Growth, Maturation and Resistance Training on Muscle-Tendon and Neuromuscular Adaptations: A Narrative Review

The purpose of this article is to provide an overview of the growth, maturation and resistance training-related changes in muscle-tendon and neuromuscular mechanisms in youth, and the subsequent effect on performance. Sprinting, jumping, kicking, and throwing are common movements in sport that have been shown to develop naturally with age, with improvements in performance being attributed to growth and maturity-related changes in neuromuscular mechanisms. These changes include moderate to very large increases in muscle physiological cross-sectional area (CSA), muscle volume and thickness, tendon CSA and stiffness, fascicle length, muscle activation, pre-activation, stretch reflex control accompanied by large reductions in electro-mechanical delay and co-contraction. Furthermore, a limited number of training studies examining neuromuscular changes following four to 20 weeks of resistance training have reported trivial to moderate differences in tendon stiffness, muscle CSA, muscle thickness, and motor unit activation accompanied by reductions in electromechanical delay (EMD) in pre-pubertal children. However, the interaction of maturity- and training-related neuromuscular adaptions remains unclear. An understanding of how different neuromuscular mechanisms adapt in response to growth, maturation and training is important in order to optimise training responsiveness in youth populations. Additionally, the impact that these muscle-tendon and neuromuscular changes have on force producing capabilities underpinning performance is unclear.

Functional relevance of resistance training-induced neuroplasticity in health and disease

Repetitive, monotonic, and effortful voluntary muscle contractions performed for just a few weeks, i.e., resistance training, can substantially increase maximal voluntary force in the practiced task and can also increase gross motor performance. The increase in motor performance is often accompanied by neuroplastic adaptations in the central nervous system. While historical data assigned functional relevance to such adaptations induced by resistance training, this claim has not yet been systematically and critically examined in the context of motor performance across the lifespan in health and disease. A review of muscle activation, brain and peripheral nerve stimulation, and imaging data revealed that increases in motor performance and neuroplasticity tend to be uncoupled, making a mechanistic link between neuroplasticity and motor performance inconclusive. We recommend new approaches, including causal mediation analytical and hypothesis-driven models to substantiate the functional relevance of resistance training-induced neuroplasticity in the improvements of gross motor function across the lifespan in health and disease.

Effectiveness and time-course adaptation of resistance training vs. plyometric training in prepubertal soccer players

PURPOSE

This study aimed to assess the effectiveness and time course for improvements in explosive actions through resistance training (RT) vs. plyometric training (PT) in prepubertal soccer players.

METHODS

Thirty-four male subjects were assigned to: a control group (n = 11); an RT group (5 regular soccer training sessions per week, n = 12); a PT group (3 soccer training sessions and 2 RT sessions per week, n = 11). The outcome measures included tests for the assessment of muscle strength (e.g., 1 repetition maximum half-squat test), jump ability (e.g., countermovement jump, squat jump, standing long jump, and multiple 5 bounds test), linear speed (e.g., 20 m sprint test), and change of direction (e.g., Illinois change of direction test).

RESULTS

The RTG showed an improvement in the half-squat (Δ = 13.2%; d = 1.3, p< 0.001) and countermovement jump (Δ = 9.4%; d = 2.4, p< 0.001) at Week 4, whereas improvements in the 20-m sprint (Δ = 4.2%; d = 1.1, p < 0.01); change of direction (CoD) (Δ = 3.8%; d = 2.1, p < 0.01); multiple 5 bounds (Δ = 5.1%; d = 1.5, p < 0.05); standing long jump (Δ = 7.2%; d = 1.2, p < 0.01); squat jump (Δ = 19.6%; d = 1.5, p < 0.01); were evident at Week 8. The PTG showed improvements in CoD (Δ = 2.1%; d = 1.3, p< 0.05); standing long jump (Δ = 9.3%; d = 1.1, p< 0.01); countermovement jump (Δ = 16.1%; d = 1.2, p< 0.01); and squat jump (Δ = 16.7%; d = 1.4, p< 0.01); at Week 8 whereas improvements in the 20-m sprint (Δ = 4.1%; d = 1.3, p < 0.01); and multiple 5 bounds (Δ = 7.4%; d = 2.4, p< 0.001); were evident only after Week. The RT and PT groups showed improvements in all sprint, CoD, and jump tests (p< 0.05) and in half-squat performance, for which improvement was only shown within the RTG (p< 0.001).

CONCLUSION

RT and PT conducted in combination with regular soccer training are safe and feasible interventions for prepubertal soccer players. In addition, these interventions were shown to be effective training tools to improve explosive actions with different time courses of improvements, which manifested earlier in the RTG than in the PTG. These outcomes may help coaches and fitness trainers set out clear and concise goals of training according to the specific time course of improvement difference between RT and PT on proxies of athletic performance of prepubertal soccer players.

Effects of long-term athletic training on muscle morphology and tendon stiffness in preadolescence: association with jump performance

Purpose

Evidence on training-induced muscle hypertrophy during preadolescence is limited and inconsistent. Possible associations of muscle strength and tendon stiffness with jumping performance are also not investigated. We investigated the thickness and pennation angle of the gastrocnemius medialis muscle (GM), as indicators for potential muscle hypertrophy in preadolescent athletes. Further, we examined the association of triceps surae muscle–tendon properties with jumping performance.

Methods

Eleven untrained children (9 years) and 21 similar-aged artistic gymnastic athletes participated in the study. Muscle thickness and pennation angle of the GM were measured at rest and muscle strength of the plantar flexors and Achilles tendon stiffness during maximum isometric contractions. Jumping height in squat (SJ) and countermovement jumps (CMJ) was examined using a force plate. We evaluated the influence of normalised muscle strength and tendon stiffness on jumping performance with a linear regression model.

Results

Muscle thickness and pennation angle did not differ significantly between athletes and non-athletes. In athletes, muscle strength was greater by 25% and jumping heights by 36% (SJ) and 43% (CMJ), but Achilles tendon stiffness did not differ between the two groups. The significant predictor for both jump heights was tendon stiffness in athletes and normalised muscle strength for the CMJ height in non-athletes.

Conclusion

Long-term artistic gymnastics training during preadolescence seems to be associated with increased muscle strength and jumping performance but not with training-induced muscle hypertrophy or altered tendon stiffness in the plantar flexors. Athletes benefit more from tendon stiffness and non-athletes more from muscle strength for increased jumping performance.

The effect of four weeks of plyometric training on reactive strength index and leg stiffness is sport dependent

BACKGROUND

Plyometric exercises are often used to develop lower limb strength and performance-related biomechanics such as leg stiffness. However, the effectiveness of plyometric training may depend on participants' own training and performance demands. The purpose of this study was to examine the effect of plyometric training on Reactive Strength Index (RSI) and leg stiffness (Kleg) on young athletes of different sports.

METHODS

Forty eight female athletes (25 Taekwondo (TKD) and 23 rhythmic gymnastics (RG), mean±SD: age: 8.94±2.50 years; mass: 29.73±7.69 kg; height: 138.84±11.90 cm; training experience: 4.62±2.37 years) participated in this study. Participants were randomly assigned to experimental (PT, N.=24) and control (CG, N.=24) groups. The PT group followed a twice-weekly plyometric training program for 4 weeks. Plyometric drills lasted approximately 5-10 s, and at least 90 s rest was allowed after each set. To examine RSI, participants performed trials of five maximal CMJs. Submaximal hopping (20 hops) was performed in order to examine leg stiffness.

RESULTS

Significant interaction effect was found for RSI and the post hoc analysis showed that RSI significantly increased by 35% (P=0.017) in RG athletes, whereas a significantly reduction by 28% (P=0.004) was revealed in TKD athletes. The interaction effect between time and group was statistically significant for Kleg (P<0.05) with Kleg significantly increasing by 31% (P=0.008) in TKD athletes, but remaining unchanged (P>0.05) in RG athletes.

CONCLUSIONS

The results showed that the effect of a 4-week plyometric training program on RSI and leg stiffness is sport dependent. Further, the applied plyometric program was effective in reducing ground contact time and therefore increasing leg stiffness.

Muscle and Tendon Morphology in Early-Adolescent Athletes and Untrained Peers

Adolescent athletes can feature significantly greater muscle strength and tendon stiffness compared to untrained peers. However, to date, it is widely unclear if radial muscle and tendon hypertrophy may contribute to loading-induced adaptation at this stage of maturation. The present study compares the morphology of the vastus lateralis (VL) and the patellar tendon between early-adolescent athletes and untrained peers. In 14 male elite athletes (A) and 10 untrained controls (UC; 12–14 years of age), the VL was reconstructed from full muscle segmentations of magnetic resonance imaging (MRI) sequences and ultrasound imaging was used to measure VL fascicle length and pennation angle. The physiological cross-sectional area (PCSA) of the VL was calculated by dividing muscle volume by fascicle length. The cross-sectional area (CSA) of the patellar tendon was measured over its length based on MRI segmentations as well. Considering body mass as covariate in the analysis, there were no significant differences between groups considering the VL anatomical cross-sectional area (ACSA) over its length or maximum ACSA (UC: 24.0 ± 8.3 cm², A: 28.1 ± 5.3 cm², p > 0.05), yet athletes had significantly greater VL volume (UC: 440 ± 147 cm³, A: 589 ± 121 cm³), PCSA (UC: 31 ± 9 cm², A: 46 ± 9 cm²), pennation angle (UC: 8.2 ± 1.4°, A: 10.1 ± 1.3°), and average patellar tendon CSA (UC: 1.01 ± 0.18 cm², A: 1.21 ± 0.18 cm²) compared to the untrained peers (p < 0.05). However, the ratio of average tendon CSA to VL PCSA was significantly lower in athletes (UC: 3.4 ± 0.1%, A: 2.7 ± 0.5%; p < 0.05). When inferring effects of athletic training based on the observed differences between groups, these results suggest that both muscle and tendon of the knee extensors respond to athletic training with radial growth. However, the effect seems to be stronger in the muscle compared to the tendon, with an increase of pennation angle contributing to the marked increase of muscle PCSA. A disproportionate response to athletic training might be associated with imbalances of muscle strength and tendon stiffness and could have implications for the disposition towards tendon overuse injury.

Effects of Different Plyometric Training Frequencies on Measures of Athletic Performance in Prepuberal Male Soccer Players

Bouguezzi, R, Chaabene, H, Negra, Y, Ramirez-Campillo, R, Jlalia, Z, Mkaouer, B, and Hachana, Y. Effects of different plyometric training frequencies on measures of athletic performance in prepuberal male soccer players. J Strength Cond Res 34(6): 1609-1617, 2020-This study aimed to compare the effects of 1 vs. 2 sessions of equal-weekly volume plyometric training (PT) across 8 weeks on measures of athletic performance (i.e., sprint time, change of direction [CoD], jumping ability, and muscle strength) in prepuberal male soccer players. Thirty participants were randomly assigned either to 1 session PT group (1SPT [n = 15]) or 2 sessions PT group (2SPT [n = 15]). Plyometric training was integrated into their regular soccer training routine. Pretraining and posttraining tests for the assessment of sprint time (e.g., 5, 10, 20, and 30-m), CoD (e.g., T-test and modified Illinois change of direction test [MICODT]), jumping ability (e.g., standing long jump [SLJ], countermovement jump [CMJ], and squat jump [SJ]), muscle strength (reactive strength index [RSI]), and kicking distance were conducted. Results showed a main effect of time for 5-m sprint-time performance (F(1,56) = 4.00, effect size [ES] = 0.53 [medium], p = 0.05), T-test (F(1,56) = 23.19, ES = 1.28 [large], p < 0.001), MICODT (F(1,56) = 5.72, ES = 0.94 [large], p = 0.02), SLJ (F(1,56) = 16.63, ES = 1.09 [large], p < 0.001), CMJ (F(1,56) = 15.43, ES = 1.04 [large], p < 0.001), SJ (F(1,56) = 20.27, ES = 1.20 [large], p < 0.001), RSI (F(1,56) = 26.26, ES = 1.36 [large], p < 0.001), and kicking distance (F(1,56) = 47.19, ES = 1.83 [large], p < 0.001). There were no training group × time interactions in all the measured outcomes. In conclusion, when an equated moderate volume of jumps is performed, higher PT frequency across 8 weeks has no extra effects on prepuberal male soccer players' measures of athletic performance. The present findings may help optimizing PT interventions dedicated to prepuberal male soccer players.

Resistance Training for Children and Adolescents

Resistance training is becoming more important as an integral part of comprehensive sport training regimens, school physical education classes, and after-school fitness programs. The increasing number of youth who are involved in sport activities, coupled with the health problems of inactivity and being overweight, have resulted in increased interest in resistance training. Secular declines in measures of muscular fitness in modern-day youth highlight the need for participation in youth resistance training for nonathletes as well as athletes. Parents often ask pediatricians to offer advice regarding the safety, benefits, and implementation of an effective resistance-training program. This report is a revision of the 2008 American Academy of Pediatrics policy statement and reviews current information and research on the benefits and risks of resistance training for children and adolescents.

Factors Influencing the Relationship Between the Functional Movement Screen and Injury Risk in Sporting Populations: A Systematic Review and Meta-analysis

BACKGROUND

Studies investigating the association between the Functional Movement Screen (FMS) and sports injury risk have reported mixed results across a range of athlete populations.

OBJECTIVES

The purpose of this systematic review was to identify whether athlete age, sex, sport type, injury definition and mechanism contribute to the variable findings.

STUDY DESIGN

Systematic review and meta-analysis.

METHODS

A systematic search was conducted in October 2018 using PubMed, EBSCOhost, Scopus, EmBase and Web of Science databases. Studies were included if they were peer reviewed and published in English language, included athletes from any competition level, performed the FMS at baseline to determine risk groups based on FMS composite score, asymmetry or pain, and prospectively observed injury incidence during training and competition. Study eligibility assessment and data extraction was performed by two reviewers. Random effects meta-analyses were used to determine odds ratio (OR), sensitivity and specificity with 95% confidence intervals. Sub-group analyses were based on athlete age, sex, sport type, injury definition, and injury mechanism.

RESULTS

Twenty-nine studies were included in the FMS composite score meta-analysis. There was a smaller effect for junior (OR = 1.03 [0.67-1.59]; p = 0.881) compared to senior athletes (OR = 1.80 [1.17-2.78]; p = 0.008) and for male (OR = 1.79 [1.08-2.96]; p = 0.024) compared to female (OR = 1.92 [0.43-8.56]; p = 0.392) athletes. FMS composite scores were most likely to be associated with increased injury risk in rugby (OR = 5.92 [1.67-20.92]; p = 0.006), and to a lesser extent American football (OR = 4.41 [0.94-20.61]; p = 0.059) and ice hockey (OR = 3.70 [0.89-15.42]; p = 0.072), compared to other sports. Specificity values were higher than sensitivity values for FMS composite score. Eleven studies were included in the FMS asymmetry meta-analysis with insufficient study numbers to generate sport type subgroups. There was a larger effect for senior (OR = 1.78 [1.16-2.73]; p = 0.008) compared to junior athletes (OR = 1.21 [0.75-1.96]; p = 0.432). Sensitivity values were higher than specificity values for FMS asymmetry. For all FMS outcomes, there were minimal differences across injury definitions and mechanisms. Only four studies provided information about FMS pain and injury risk. There was a smaller effect for senior athletes (OR = 1.28 [0.33-4.96]; p = 0.723) compared to junior athletes (OR = 1.71 [1.16-2.50]; p = 0.006). Specificity values were higher than sensitivity values for FMS pain.

CONCLUSION

Athlete age, sex and sport type explained some of the variable findings of FMS prospective injury-risk studies. FMS composite scores and asymmetry were more useful for estimating injury risk in senior compared to junior athletes. Effect sizes tended to be small except for FMS composite scores in rugby, ice hockey and American football athletes.

PROTOCOL REGISTRATION

CRD42018092916.

Effects of Suspension Versus Traditional Resistance Training on Explosive Strength in Elementary School-Aged Boys

PURPOSE

The aim of this study was to determine the effects of an 8-week program of resistance training (RT) or suspension training (ST) on explosive strength in prepubescent boys.

METHODS

Fifty-seven boys aged 10-11 years were assigned to 2 training groups, RT or ST or a control group (no training program). Boys trained twice weekly for 8 weeks.

RESULTS

A significant interaction was reported with a large (P < .001, ηp2=.463), medium (P < .001, ηp2=.395), and small effect sized (P ≤ .001, ηp2=.218) in the 1-kg ball throw, 3-kg ball throw, and time-at-20-m test, respectively. There was no significant interaction in the countermovement vertical jump or the standing long jump. Changes from preintervention to postintervention for the 1-kg ball throw were 5.94% and 5.82% for the ST and RT, respectively, and 8.82% and 8.14% in the 3-kg ball throw for the ST and RT, respectively. The improvement in the 20-m sprint was 1.19% for the ST and 2.33% for the RT.

CONCLUSION

Traditional RT and ST seem to be effective methods for improving explosive strength in prepubescent boys. ST could be considered as an alternative modality to optimize explosive strength training in school-based programs.

Triceps Surae Muscle-Tendon Unit Properties in Preadolescent Children: A Comparison of Artistic Gymnastic Athletes and Non-athletes

Knowledge regarding the effects of athletic training on the properties of muscle and tendon in preadolescent children is scarce. The current study compared Achilles tendon stiffness, plantar flexor muscle strength and vertical jumping performance of preadolescent athletes and non-athletes to provide insight into the potential effects of systematic athletic training. Twenty-one preadolescent artistic gymnastic athletes (9.2 ± 1.6 years, 15 girls) and 11 similar-aged non-athlete controls (9.0 ± 1.7 years, 6 girls) participated in the study. The training intensity and volume of the athletes was documented for the last 6 months before the measurements. Subsequently, vertical ground reaction forces were measured with a force plate to assess jumping performance during squat (SJ) and countermovement jumps (CMJ) in both groups. Muscle strength of the plantar flexor muscles and Achilles tendon stiffness were examined using ultrasound, electromyography, and dynamometry. The athletes trained 6 days per week with a total of 20 h of training per week. Athletes generated significantly greater plantar flexion moments normalized to body mass compared to non-athletes (1.75 ± 0.32 Nm/kg vs. 1.31 ± 0.33 Nm/kg; p = 0.001) and achieved a significantly greater jump height in both types of jumps (21.2 ± 3.62 cm vs. 14.9 ± 2.32 cm; p < 0.001 in SJ and 23.4 ± 4.1 cm vs. 16.4 ± 4.1 cm; p < 0.001 in CMJ). Achilles tendon stiffness did not show any statistically significant differences (p = 0.413) between athletes (116.3 ± 32.5 N/mm) and non-athletes (106.4 ± 32.8 N/mm). Athletes were more likely to reach strain magnitudes close to or higher than 8.5% strain compared to non-athletes (frequency: 24% vs. 9%) indicating an increased mechanical demand for the tendon. Although normalized muscle strength and jumping performance were greater in athletes, gymnastic-specific training in preadolescence did not cause a significant adaptation of Achilles tendon stiffness. The potential contribution of the high mechanical demand for the tendon to the increasing risk of tendon overuse call for the implementation of specific exercises in the athletic training of preadolescent athletes that increase tendon stiffness and support a balanced adaptation within the muscle-tendon unit.

Muscle strength, size, and neuromuscular function before and during adolescence

PURPOSE

To compare measurements of muscle strength, size, and neuromuscular function among pre-adolescent and adolescent boys and girls with distinctly different strength capabilities.

METHODS

Fifteen boys (mean age ± confidence interval: 13.0 ± 1.0 years) and 13 girls (12.9 ± 1.1 years) were categorized as low strength (LS, n = 14) or high strength (HS, n = 14) based on isometric maximal voluntary contraction strength of the leg extensors. Height (HT), seated height, and weight (WT) determined maturity offset, while percent body fat and fat-free mass (FFM) were estimated from skinfold measurements. Quadriceps femoris muscle cross-sectional area (CSA) was assessed from ultrasound images. Isometric ramp contractions of the leg extensors were performed while surface electromyographic amplitude (EMG_RMS) and mechanomyographic amplitude (MMG_RMS) were recorded for the vastus lateralis (VL). Neuromuscular efficiency from the EMG and MMG signals (NME_EMG and NME_MMG, respectively) and log-transformed EMG and MMG vs. torque relationships were also used to examine neuromuscular responses.

RESULTS

HS was 99-117% stronger, 2.3-2.8  years older, 14.0-15.7 cm taller, 20.9-22.3 kg heavier, 2.3-2.4 years more biologically mature, and exhibited 39-43% greater CSA than LS (p ≤ 0.001). HS exhibited 74-81% higher NME_EMG than LS (p ≤ 0.022), while HS girls exhibited the highest NME_MMG (p ≤ 0.045). Even after scaling for HT, WT, CSA, and FFM, strength was still 36-90% greater for HS than LS (p ≤ 0.031). The MMG_RMS patterns in the LS group displayed more type I motor unit characteristics.

CONCLUSIONS

Neuromuscular adaptations likely influence strength increases from pre-adolescence to adolescence, particularly when examining large, force-producing muscles and large strength differences explained by biological maturity, rather than simply age.

Strength Profile of Hip Abductor and Adductor Muscles in Youth Elite Soccer Players

The main objective of this study was to provide an extensive isokinetic profile of the hip joint in youth soccer players, where the literature is limited. Additionally, this study investigated the effect of age on isokinetic peak torque values of hip abductor and adductor muscles and on reciprocal muscle group torque ratios in youth soccer players at different angular velocities (30 vs. 90o/s) and muscle actions (concentric vs. eccentric). Sixty young elite male soccer players were assigned into three equal groups (n = 20): children, young adolescents and older adolescents, and performed five maximal concentric and eccentric hip-abductions and adductions at 30o/s and 90o/s. The results showed a significant increase (p < 0.05) in peak torque values from childhood to adolescence, with the exception of young adolescents vs. older adolescents where no differences were observed. The reciprocal ratios were not affected by age, but improved with an increase in angular velocity with the exception of the CON/ECC ratio that was higher at 30o/s. The data presented in this study provide an extensive isokinetic profile of hip abductor and adductor muscle strength in youth soccer players to assist both coaches and sports medicine professionals in strength monitoring and training.

Effects of Plyometric and Resistance Training on Muscle Strength, Explosiveness, and Neuromuscular Function in Young Adolescent Soccer Players

McKinlay, BJ, Wallace, P, Dotan, R, Long, D, Tokuno, C, Gabriel, D, and Falk, B. Effects of plyometric and resistance training on muscle strength, explosiveness, and neuromuscular function in young adolescent soccer players. J Strength Cond Res 32(11): 3039-3050, 2018-This study examined the effect of 8 weeks of free-weight resistance training (RT) and plyometric (PLYO) training on maximal strength, explosiveness, and jump performance compared with no added training (CON), in young male soccer players. Forty-one 11- to 13-year-old soccer players were divided into 3 groups (RT, PLYO, and CON). All participants completed isometric and dynamic (240°·s) knee extensions before and after training. Peak torque (pT), peak rate of torque development (pRTD), electromechanical delay (EMD), rate of muscle activation (Q50), m. vastus lateralis thickness (VLT), and jump performance were examined. Peak torque, pRTD, and jump performance significantly improved in both training groups. Training resulted in significant (p ≤ 0.05) increases in isometric pT (23.4 vs. 15.8%) and pRTD (15.0 vs. 17.6%), in RT and PLYO, respectively. During dynamic contractions, training resulted in significant increases in pT (12.4 and 10.8% in RT and PLYO, respectively), but not in pRTD. Jump performance increased in both training groups (RT = 10.0% and PLYO = 16.2%), with only PLYO significantly different from CON. Training resulted in significant increases in VLT (RT = 6.7% and PLYO = 8.1%). There were no significant EMD changes. In conclusion, 8-week free-weight resistance and plyometric training resulted in significant improvements in muscle strength and jump performance. Training resulted in similar muscle hypertrophy in the 2 training modes, with no clear differences in muscle performance. Plyometric training was more effective in improving jump performance, whereas free-weight RT was more advantageous in improving peak torque, where the stretch reflex was not involved.

Protein Metabolism in Active Youth: Not Just Little Adults

Understanding how exercise and dietary protein alter the turnover and synthesis of body proteins in youth can provide guidelines for the optimal development of lean mass. This review hypothesizes that active youth obtain similar anabolic benefits from exercise and dietary protein as adults, but the requirement for amino acids to support growth renders them more sensitive to these nutrients.

Effect of a Suspension-Trainer-Based Movement Program on Measures of Fitness and Functional Movement in Children: A Pilot Study

PURPOSE

The purpose of this pilot study was to assess the efficacy of a suspension-training movement program to improve muscular- and skill-related fitness and functional movement in children, compared with controls.

METHODS

In total, 28 children [male: 46%; age: 9.3 (1.5) y; body mass index percentile: 68.6 (27.5)] were randomly assigned to intervention (n = 17) or control (n = 11) groups. The intervention group participated in a 6-week suspension-training movement program for two 1-hour sessions per week. Muscular- and skill-related fitness and functional movement assessments were measured at baseline and following the intervention. Analyses of covariance models were used to assess the effects of time and intervention.

RESULTS

The intervention participants achieved greater improvements in Modified Pull-Up performance (P = .01, Cohen's d = 0.54) and Functional Movement Screen score (P < .001, Cohen's d = 1.89), relative to controls.

CONCLUSION

The suspension-training intervention delivered twice a week was beneficial for upper body pulling muscular endurance and the Functional Movement Screen score. Future interventions using this modality in youth would benefit from larger, more diverse samples (through schools or community fitness centers) and a longer intervention length.

Maturation-Related Differences in Adaptations to Resistance Training in Young Male Swimmers

Moran, J, Sandercock, GRH, Ramírez-Campillo, R, Wooller, J-J, Logothetis, S, Schoenmakers, PPJM, and Parry, DA. Maturation-related differences in adaptations to resistance training in young male swimmers. J Strength Cond Res 32(1): 139-149, 2018-This study examined the effects of resistance training on muscular strength and jump performances in young male swimmers. It was hypothesized that adaptations would be of a lower magnitude in less mature (prepeak height velocity [PHV]) than in more mature (post-PHV) subjects. Fourteen pre-PHV (-1.8 ± 1.0 years) and 8 post-PHV (1.6 ± 0.5 years) swimmers undertook a 30 minutes, twice-weekly resistance training program for 8 weeks. They were compared with matched control groups (pre-PHV: -2.0 ± 1.1, n = 15; post-PHV: 1.2 ± 1.0, n = 7). The effects on lower-body isometric strength (LBS), measured with midthigh pull, and vertical jump (VJ) height in the post-PHV group were large (effect size: 1.3 [0.4 to 2.2]) and small (0.4 [-0.4 to 1.2]), respectively. Effects on LBS and VJ height in the pre-PHV group were moderate (0.8 [0.1 to 1.4]) and trivial (0.2 [-0.5 to 0.8]), respectively. Estimates in the post-PHV control group (LBS: 0.7 [-0.2 to 1.6]; VJ: 0.2 [-0.7 to 1.0]) and the pre-PHV control group (LBS: 0.1 [-0.5 to 0.7]; VJ: -0.3 [-0.9 to 0.3]) may indicate the extent to which maturation could contribute to the performance changes seen in the respective training groups. Lower-body isometric strength and VJ are trainable, but to different magnitudes, in pre- and post-PHV swimmers. After appropriate foundational training to establish technical competency, twice-weekly resistance training sessions of 30 minutes duration, comprising 3 sets of 4 exercises can be effective in pre- and post-PHV youth.

Effects of Plyometric Training on Components of Physical Fitness in Prepuberal Male Soccer Athletes: The Role of Surface Instability

Negra, Y, Chaabene, H, Sammoud, S, Bouguezzi, R, Mkaouer, B, Hachana, Y, and Granacher, U. Effects of plyometric training on components of physical fitness in prepuberal male soccer athletes: the role of surface instability. J Strength Cond Res 31(12): 3295-3304, 2017-Previous studies contrasted the effects of plyometric training (PT) conducted on stable vs. unstable surfaces on components of physical fitness in child and adolescent soccer players. Depending on the training modality (stable vs. unstable), specific performance improvements were found for jump (stable PT) and balance performances (unstable PT). In an attempt to combine the effects of both training modalities, this study examined the effects of PT on stable surfaces compared with combined PT on stable and unstable surfaces on components of physical fitness in prepuberal male soccer athletes. Thirty-three boys were randomly assigned to either a PT on stable surfaces (PTS; n = 17; age = 12.1 ± 0.5 years; height = 151.6 ± 5.7 cm; body mass = 39.2 ± 6.5 kg; and maturity offset = -2.3 ± 0.5 years) or a combined PT on stable and unstable surfaces (PTC; n = 16; age = 12.2 ± 0.6 years; height = 154.6 ± 8.1 cm; body mass = 38.7 ± 5.0 kg; and maturity offset = -2.2 ± 0.6 years). Both intervention groups conducted 4 soccer-specific training sessions per week combined with either 2 PTS or PTC sessions. Before and after 8 weeks of training, proxies of muscle power (e.g., countermovement jump [CMJ], standing long jump [SLJ]), muscle strength (e.g., reactive strength index [RSI]), speed (e.g., 20-m sprint test), agility (e.g., modified Illinois change of direction test [MICODT]), static balance (e.g., stable stork balance test [SSBT]), and dynamic balance (unstable stork balance test [USBT]) were tested. An analysis of covariance model was used to test between-group differences (PTS vs. PTC) at posttest using baseline outcomes as covariates. No significant between-group differences at posttest were observed for CMJ (p > 0.05, d = 0.41), SLJ (p > 0.05, d = 0.36), RSI (p > 0.05, d = 0.57), 20-m sprint test (p > 0.05, d = 0.06), MICODT (p > 0.05, d = 0.23), and SSBT (p > 0.05, d = 0.20). However, statistically significant between-group differences at posttest were noted for the USBT (p < 0.01, d = 1.49) in favor of the PTC group. For most physical fitness tests (except RSI), significant pre-to-post improvements were observed for both groups (p < 0.01, d = 0.55-3.96). Eight weeks of PTS or PTC resulted in similar performance improvements in components of physical fitness except for dynamic balance. From a performance-enhancing perspective, PTC is recommended for pediatric strength and conditioning coaches because it produced comparable training effects as PTS on proxies of muscle power, muscle strength, speed, agility, static balance, and additional effects on dynamic balance.

Rehabilitation Following Anterior Cruciate Ligament Tears in Children: A Systematic Review

BACKGROUND

Anterior cruciate ligament (ACL) tears are increasingly prevalent in the pediatric population. ACL rehabilitation is an essential component of recovery following injury and reconstruction, yet there are few explicit descriptions of pediatric-specific ACL rehabilitation protocols in the literature, especially in the context of varying treatment interventions. Our aim was to systematically review the literature on rehabilitation following ACL tears in children in order to describe common principles among different treatment options and areas of future research.

METHODS

Using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, we performed a systematic review of the PubMed, EMBASE, and Cochrane databases (for the past five years) to identify detailed rehabilitation protocols described in the pediatric population following ACL rupture. When available, the following aspects of rehabilitation were extracted: "prehabilitation" (exercises prior to surgery), bracing, weight-bearing status, range of motion, strength, modalities (ice, heat, electrical stimulation, etc.), plyometrics/proprioceptive exercises, return-to-sport criteria, and suggested ACL injury-prevention programs.

RESULTS

Two hundred and two unique articles were identified. Twenty-seven articles meeting inclusion criteria with extractible rehabilitation data were included. A table, categorized by differing orthopaedic intervention, was designed to detail the components and duration of the different aspects of rehabilitation. While there are substantial differences across protocols, several trends emerged, particularly regarding weight-bearing, bracing, range of motion, and strength training. Interestingly, we found that many current protocols are based on time frame alone rather than on functional milestones; of the fourteen unique articles that addressed return-to-sport criteria by specific orthopaedic intervention, seven were based on temporal progression whereas seven also involved achievement of physical milestones. In addition, only three of the eight articles that mentioned a future ACL injury-prevention plan described a formal prevention program.

CONCLUSION

We systematically identified, and subsequently outlined and compared, the current trends of the various components of pediatric-specific ACL rehabilitation protocols, categorized by orthopaedic intervention. Several protocols are based on time frames rather than milestones achieved, with newer protocols involving milestone-based progression. Newer protocols are also incorporating formal prevention programs. Just as skeletally immature patients require unique methods of operative fixation, so too do they require catered rehabilitation protocols. To effectively prevent re-rupture or contralateral injury, future research should focus on prospectively evaluating each component of the rehabilitation protocols described and return-to-sport criteria for young patients.

Evidence of homologous and heterologous effects after unilateral leg training in youth

The positive effects of unilateral training on contralateral muscles (cross education) has been demonstrated with adults for over a century. There is limited evidence for cross education of heterologous muscles. Cross education has not been demonstrated with children. It was the objective of this study to investigate cross-education training in children examining ipsilateral and contralateral homologous and heterologous muscles. Forty-eight male children (aged 10-13 years) were assessed for unilateral, ipsilateral and contralateral lower limb strength, power and endurance (1-repetition maximum (RM) leg press, knee extensors (KE) and flexors (KF) maximum voluntary isometric contractions (MVIC), countermovement jump, muscle endurance test (leg press repetitions with 60% 1RM)), and upper body unilateral MVIC elbow flexors (EF) and handgrip strength. An 8-week training program involved 2 unilateral leg press resistance-training groups (high load/low repetitions: 4-8 sets of 5RM, and low load/high repetitions: 1-2 sets of 20RM) and control (untrained) group. All muscles exhibited improvements of 6.1% to 89.1%. The trained limb exhibited greater adaptations than the untrained limb for leg press 1RM (40.3% vs. 25.2%; p = 0.005), and 60% 1RM leg press (104.1% vs. 73.4%; p = 0.0001). The high load/low repetition training induced (p < 0.0001) greater improvements than low load/high repetition with KE, KF, EF MVIC and leg press 1RM. This is the first study to demonstrate cross-education effects with children and that the effects of unilateral training involve both contralateral homologous and heterologous muscles with the greatest strength-training responses from high-load/low-repetition training.